Secondary-side amplifier with soft start

ABSTRACT

An isolated switching regulator has a closed-loop soft-start feature that allows tighter regulation of the output voltage and eliminates or reduces overshoot. It also has an optional reset feature which will resoft-start the regulator during recovery from a fault on the output voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. application Ser. No.60/980,691 filed on Oct. 17, 2007, which is incorporated herein in itsentirety by reference.

FIELD OF THE INVENTION

This application relates to isolated switching regulators and morespecifically to a soft-start circuit for such regulators.

BACKGROUND OF THE INVENTION

Isolated switching regulators are commonly used in consumer devices withhigh input voltages to convert them to lower voltages required bysubsystems and safely separate users from hazardous high voltages. Theisolation is usually provided by using transformer in power stage andopto-coupler in feedback path as shown in FIG. 1 for an example of anisolated flyback topology. The circuit is therefore divided to primaryand secondary sides. While primary side contains gate drive, PWMcomparator, and over current protection circuits, secondary-side circuitcontains error amplifier and reference voltage. However, because ofisolation required, secondary and primary side cannot be integrated in asingle integrated circuit (IC).

Traditionally, a discrete solution such as an adjustable shunt regulatorcircuit (TL431 family) is used to function as the secondary-sidecontroller (i.e., a bandgap and an error amplifier). FIG. 2 shows how ashunt regulator circuit is typically used as the secondary sidecontroller in an isolated converter such as an isolated flyback. In thiscircuit, resistors R₁ and R₂ and capacitor C_(c) are loop compensationelements and resistor R₃ is used to set the output voltage (i.e.,V_(out)=(1+R₁/R₃)V_(BG), where V_(BG) is 1.24V bandgap voltage). Theprimary side usually is implemented as a current-mode PWM converter, andits basic block diagram is shown in FIG. 2. To simplify the compensationand increase its robustness, in addition to current- and voltage-loops,a feed-forward path is added through opto-coupler input diode anode.More detailed information on compensation and secondary-side controlwith shunt regulator circuit devices is shown in FIG. 3. Althoughillustrated as a flyback converter, these shunt regulator circuits canbe used for other isolated topologies such as a forward converters.

Discrete solutions such as shown, however, can not provide closed loopsoft start to tightly control output voltage V_(out) during startupsequence. Available solutions with shunt regulators usually use openloop soft-start schemes that control some other voltage or currentduring the startup such as clamping the V_(opto) cathode voltage(V_(opto)) at startup or limiting the transformer current at primary IC.However, since these techniques control V_(out) indirectly (e.g., openloop), V_(out) may experience overshoot during startup (i.e., nonmonolithic startup). The problem worsens for lower V_(out) values. Thecathode voltage of shunt regulator (V_(opto) in FIG. 2) needs minimum of1.24V for operation, assuming 1V drop for opto coupler diode, and 0.75Von bias resistor R_(opto1), TL431 starts taking over the loop controlonly when V_(out) has reached about 3V. This can cause soft-startproblems if V_(out) final value is set at 3.3V.

It is also known to use an open-loop soft-start circuit using discretecomponents, as shown in FIG. 4. This circuit utilizes resistor R13,capacitor C18 and diode D9 to implement the open-loop soft-startcircuit. Note that this circuit does not compare the output voltage witha reference during start up, so that closed-loop control is notpossible.

SUMMARY OF THE INVENTION

It is a general object of the invention to provide an improvedsoft-start circuit for an isolated switching regulator.

This and other objects and features are attained in accordance with anaspect of the invention by an isolated switching regulator comprising aninput side of a first isolating device, the input side circuitgenerating a switched voltage across the first isolation device. Anoutput side circuit coupled to an output side of the isolating device,the output side circuit generating control signals to control the inputside circuit to generate a regulated output voltage. A second isolatingdevice coupled between the output side circuit and the input sidecircuit for conveying the control signals generated by the output sidecircuit to the input side circuit, wherein the output side circuitcomprises a closed-loop soft-start circuit.

Another aspect of the invention includes a closed-loop soft-startcircuit in an isolated switching regulator comprising a primary sidecircuit coupled via a first isolating device to a secondary sidecircuit, the secondary side circuit transmitting control signals to theprimary side circuit via a second isolating device. A closed-loopsoft-start circuit generating control signals during startup of theisolated switching regulator once an output voltage reachedsubstantially 2V_(T)+VdSSAT of the transistors comprising the soft-startcircuit where V_(T) is the threshold voltage and Vdd sat is the drain tosource voltage to place an MOS device in saturation and a secondisolating device to transmit the control signals to the primary sidecircuit for controlling the output voltage.

A further aspect comprises an isolated switching regulator comprising aprimary winding of a transformer and generating a switched voltageacross the transformer primary winding for generating a regulatedvoltage at an output of the switching regulator. A secondary sidecircuit coupled to a secondary winding of the transformer for generatingcontrol signals to control the primary side circuit to generate theregulated voltage at the output of the switching regulator. Anopto-coupler coupled to a control input of the primary side circuit anda control output of the secondary side circuit for transmitting thecontrol signals from the secondary side circuit to the primary sidecircuit. A closed-loop soft-start circuit able to start closed-loopcontrol of the primary side circuit to regulate the output voltage whenthe output voltage reaches substantially 2V_(T)+Vd ssat of thetransistors of the soft-start circuit, where V_(T) equals the thresholdvoltage and VdSSAT equals the drain to source voltage to place an MOSdevice in saturation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an isolated switching regulator;

FIG. 2 is a block diagram of a prior art isolated regulator having anopen-loop soft-start circuit;

FIG. 3 is a schematic diagram of the circuit shown in the block diagramof FIG. 2;

FIG. 4 shows an open-loop soft-start circuit;

FIG. 5 is a schematic diagram of an isolated switching regulatoraccording to the present invention;

FIG. 6 is a more detailed schematic of the circuit generating thesecondary side soft-start ramp;

FIG. 7 is a schematic diagram of the error amplifier circuit;

FIG. 8 shows the secondary side circuit integrated within a powermanagement integrated circuit;

FIG. 9 shows a circuit implementation of a dual reference;

FIG. 10 shows the output and soft-start pin voltages; and

FIG. 11 shows a stable plateau region.

DETAILED DESCRIPTION OF THE INVENTION

Our solution proposes a closed loop soft-start scheme to tightly controlthe output voltage at startup. The circuit consists of an erroramplifier Amp1, pull down transistor M_(out), a low-voltage bandgapcircuit, and a minimum detection circuit choosing between the lower ofsoft-start ramp and bandgap reference (FIGS. 5 and 6). Since supplyvoltage for BG and error amplifier (V_(supply)) and pull down transistoroutput (V_(opto)) are separate in proposed topology and V_(opto) can bedriven close to zero, the circuit can start regulating for V_(out) aslow as 1.8V (worst case) or as low as 1.5V (typical).

The soft-start procedure is as follows. Before V_(in) is applied toprimary IC, V_(out) is at zero. When V_(in) is applied to primary,primary FB node is high through a pull down resistor, since there is nocontrol yet from secondary side. Therefore, primary side IC startsswitching transistor M₁ with high duty cycle. Usually, a current limitfunction is placed in primary IC to limit the current at startup. Outputvoltage V_(out) starts to rise with almost no control from secondaryside until the voltage is raised enough for secondary side to regulate(about 1.8V). At this point, error amplifier Amp₁ tries to regulateFB_(SS) node to minimum of bandgap voltage V_(REF) and soft-start rampV_(SS). In early phase of startup, V_(SS) is lower than V_(REF) andsecondary side regulates the output voltage to V_(SS). Since V_(SS)starts from zero, V_(opto) is pulled down to ground and FB node ispulled low. However, the loop usually is designed such that FB node atthe input of primary circuit does not reach zero even when opto input ispulled to the ground, and circuit finds a stable point through itscurrent feedback loop where V_(out) sits at a voltage V_(plateau), whichis a function of loop components specifically R_(opto2) This isdescribed below in connection with FIG. 11. Output voltage V_(out) sitsat V_(plateau) until V_(SS) rises above V_(plateau)(R₃/(R₃+R₁)). At thispoint, V_(out) starts to rise again since FB_(SS) is following risingV_(SS). Once V_(SS) goes above V_(REF), FB_(SS) is regulated to V_(BG).V_(SS) rises and finally is clamped to about 2.2V.

FIG. 6 shows more of implementation details of the secondary sidesoft-start ramp. A low-voltage current source generated in low-voltagebandgap unit charges the capacitor C_(SS) once V_(supply)(V_(out)) risesenough for circuit to operate. The capacitor is discharged throughinternal leakage resistor R_(leak) when power supply is disconnected. Tostart V_(SS) at a non-zero voltage, resistors R_(SSint and) R_(SSext)are added. The shape and timing of soft-start ramp and consequentlyV_(out) startup can be adjusted by selection of C_(SS) and R_(SSext).

FIG. 7 shows circuit implementation of error amplifier, minimumfunction, and output pull down transistor. The minimum function isembedded in input stage of the error amplifier by parallelingtransistors M₂ and M_(SS). The transistor that has the lowest voltage atits gate forms the input stage with transistor M₁. Therefore, on system,FB_(SS) node is regulated to the lower of V_(REF) and V_(SS). Theamplifier is a symmetrical OTA with a common source output stageconsisting of transistor M_(out) and external resistor R_(opto1).

As another advantage, the proposed secondary side amplifier can beintegrated inside the power management IC that usually follow primaryregulator in the system (FIG. 8). In these applications, the highvoltage of source is first stepped down safely through an isolatedregulator and then this intermediate voltage is converted to voltagerails required in the system.

Integration of secondary side controller in power management IC enablesimplementation of other features. For example, reference voltage can beswitched to a more accurate reference, once V_(out) is raised enough.Usually, low voltage bandgap are less accurate and has lower powersupply rejection ratio. Therefore, the idea is to soft start the systemwith a low voltage but low accuracy bandgap and then switch to a higherperformance reference, which requires more headroom (FIG. 9).

Restarting the soft-start is another feature that can be implemented onproposed secondary side circuit once it is integrated in the system.When V_(out) has reached its final value after startup, faults such asoverload or shorts at the output can bring down the output voltage.Although circuit restarts after the fault is removed, soft start may notreinitiate since there may not be enough time for R_(leak) to dischargesoft-start capacitor. Transistor M_(RST) is added to discharge thecapacitor C_(SS) and re soft-start the converter when RE_SOFT_STARTsignal is toggled high. RE_SOFT_START signal can be generated bysupervisory circuits that monitor V_(out) such as under voltage level.

FIG. 10 shows the measurement results for an isolated flyback circuitcontrolled by the proposed secondary side amplifier and soft-start.Channel 3 shows soft start pin (V_(SSext)) and Channel 2 shows theoutput voltage waveform. Upon power up, V_(ssext) ramps from zero untilit is clamped at about 2V. As explained before, V_(out) is first rampedup by a rate controlled by current limit of primary side until itexceeds headroom requirements of secondary side circuits. At this point,secondary side takes over. Because of loop setup and since it takes timefor soft start ramp to reach initial V_(out) value, V_(out) sits onconstant voltage of V_(plateau) until V_(ss) catches up (Time period ofwhich V_(out) stays at V_(plateau) can be adjusted and totallyeliminated through selection of R_(SS)). At this point, V_(out) followssoft start ramp, and the slope is a function of C_(ss) value andsoft-start charging current. When V_(ss) exceeds bandgap voltageV_(ref), secondary side regulates FB_(SS) to V_(ref) or equivalentlyV_(out) to (1+R₁/R₃)V_(REF). The jump in V_(out) toward the end of thewaveforms is when V_(REF) is switched to more accurate bandgap.

For most of available isolated regulator loops, even when the optocoupler input-side cathode is grounded, the output voltage regulates toa stable point V_(plateau) (FIG. 10) determined by loop components. Thisphenomenon further explained for a simplified isolated loop shown inFIG. 11. The loop tries to make the signals at the input of primary sidePWM comparator equal. Therefore,R_(sense)I_(Lmax)=V_(FB),  (1)

where R_(sense) is the current-sensing gain and I_(Lmax) is thetransformer primary side current peak. The loop is usually designed in away that even when the opto coupler diode cathode is grounded, theV_(FB) node is not at zero, but is pulled down to

${V_{DD} - {\left( {V_{out} - V_{diode}} \right)\frac{R_{{opto}\; 2}}{R_{{opto}\; 1}}}},$where V_(diode) is the opto coupler input diode voltage, V_(DD) is thereference voltage from primary IC (usually 5V), and opto coupler gain isassumed to be one. Therefore, V_(out) finds a stable point to satisfythe following condition

$\begin{matrix}{{R_{sense}I_{Lmax}} = {\left\lbrack {V_{DD} - {\left( {V_{out} - V_{diode}} \right)\frac{R_{{opto}\; 2}}{R_{{opto}\; 1}}}} \right\rbrack.}} & (2)\end{matrix}$

Since for a converter working in continuous-time PWM operation, I_(Lmax)can be specified as a function of input voltage, output voltage, and theload or equivalently I_(Lmax)=f(_(Vout)). Therefore, V_(out) regulatesto a stable point determined by loop components such as current-sensinggain, VDD voltage, and opto coupler resistor ratio (i.e., V_(out) _(—)_(Plateau)=f(R_(opto2)/R_(opto1), R_(sense), V_(DD))). Other loopparameters affect the plateau voltage only as second-order effects. ForTI isolated flyback boards, R_(opto2)/R_(opto1)=2, V_(DD)=5V, andV_(out) _(—) _(Plateau)=2.4V.

Advantages of the present invention include a closed loop soft-start forisolated regulators thereby providing closer control over the outputvoltage and lower headroom requirement for secondary side amplifiercompared to prior art shunt regulators, seamless transition fromsoft-start to normal operation (i.e., implementation of minimum functionusing transistors M₂ and M_(SS) in FIG. 7). It also provides controlover duration of V_(plateau) region. Adding resistor R_(SS) in serieswith soft-start capacitor causes soft start ramp to start higher thanzero and therefore soft-start ramp reaching plateau voltage sooner.Adding enough R_(ss) can totally eliminate plateau region and result inmonolithic startup. The addition of re soft-start feature(RE_SOFT_START) and switching from a crude but low-voltage bandgap toaccurate, high performance bandgap when V_(out) has raised close to itsfinal value.

Although the present invention has been described with reference to aspecific embodiment, it is not limited to this embodiment and no doubtalternatives will occur to the skilled person that lie within the scopeof the invention as claimed.

The invention claimed is:
 1. An isolated switching regulator comprising:a input side circuit coupled to an input side of a first isolatingdevice, the input side circuit generating a switched voltage across thefirst isolation device; an output side circuit coupled to an output sideof the isolating device, the output side circuit generating controlsignals to control the input side circuit to generate a regulated outputvoltage; a second isolating device coupled between the output sidecircuit and the input side circuit for conveying the control signalsgenerated by the output side circuit to the input side circuit, whereinthe output side circuit comprises a closed-loop soft-start circuit, andwherein the closed-loop soft-start circuit comprises: an error amplifierhaving a first input coupled to a lower of a reference voltage and aone-time ramp voltage and having a second input coupled to a voltagerepresentative of the regulated output voltage, the soft-start circuitregulating the output voltage to the ramp voltage until it exceeds thereference voltage.
 2. The isolated switching regulator of claim 1wherein the first isolating device is a transformer.
 3. The isolatedswitching regulator of claim 1, wherein the second isolating device isan opto-coupler.
 4. The isolated switching regulator of claim 2, whereinthe second isolating device is an opto-coupler.
 5. The isolatedswitching regulator of claim 1 further comprising: a pull-downtransistor coupled to the second isolating device, for operating thesecond isolating device.
 6. The isolated switching regulator of claim 4further comprising: a pull-down transistor coupled to the opto-couplerfor operating the opto-coupler.
 7. An isolated switching regulatorcomprising: a input side circuit coupled to an input side of a firstisolating device, the input side circuit generating a switched voltageacross the first isolation device; an output side circuit coupled to anoutput side of the isolating device, the output side circuit generatingcontrol signals to control the input side circuit to generate aregulated output voltage; a second isolating device coupled between theoutput side circuit and the input side circuit for conveying the controlsignals generated by the output side circuit to the input side circuit,wherein the output side circuit comprises a closed-loop soft-startcircuit, and wherein the closed-loop soft-start circuit comprises: anerror amplifier having a first input coupled to a lower of a referencevoltage and a ramp voltage and having a second input coupled to avoltage representative of the regulated output voltage, wherein theclosed-loop soft-start circuit can begin control of the regulated outputvoltage when it reaches a level equal to substantially 2 V_(T)+VdSSAT ofthe transistors comprising the soft-start circuit, where V_(T) equalsthe threshold voltage and VdSSAT is the drain to source voltage to placean MOS device in saturation.
 8. The isolated switching regulator ofclaim 7 where the voltage 2 V_(T)+VdSSAT is substantially 1.6 volts. 9.The isolated switching regulator of claim 1 further comprising a resetcircuit that will cause the isolated switching regulator to resoft-startafter a fault on the output voltage.
 10. In an isolated switchingregulator comprising: a primary side circuit coupled via a firstisolating device to a secondary side circuit, the secondary side circuittransmitting control signals to the primary side circuit via a secondisolating device, a closed-loop soft-start circuit generating controlsignals during startup of the isolated switching regulator once anoutput voltage reaches substantially 2 V_(T)+VdSSAT of the transistorscomprising the soft-start circuit where V_(T) is threshold voltage andVdSSAT is the drain to source voltage to place an MOS device insaturation; and driver means to drive the second isolating device totransmit the control signals to the primary side circuit for controllingthe output voltage.
 11. The isolated switching regulator of claim 10wherein the first isolating device is a transformer.
 12. The isolatedswitching regulator of claim 10 wherein the second isolating device is aopto-coupler.
 13. The isolated switching regulator of claim 11 whereinthe second isolating device is a opto-coupler.
 14. The closed-loopsoft-start circuit of claim 10 further comprising a reset circuit thatwill cause the isolated switching regulator to resoft-start after afault on the output voltage.
 15. An isolated switching regulatorcomprising: a primary side circuit coupled to a primary winding of atransformer and generating a switched voltage across the transformerprimary winding for generating a regulated voltage at an output of theswitching regulator; a secondary side circuit coupled to a secondarywinding of the transformer for generating control signals to control theprimary side circuit to generate the regulated voltage at the output ofthe switching regulator; an opto-coupler coupled to a control input ofthe primary side circuit and a control output of the secondary sidecircuit for transmitting the control signals from the secondary sidecircuit to the primary side circuit; and a closed-loop soft-startcircuit able to start closed-loop control of the primary side circuit toregulate the output voltage when the output voltage reachessubstantially 2 V_(T)+VdSSAT of the transistors of the soft-startcircuit, where V_(T) equals the threshold voltage and VdSSAT equals thedrain to source voltage to place an MOS device in saturation.
 16. Theisolated switching regulator of claim 15 wherein 2 V_(T)+VdSSAT equalssubstantially 1.6 volts.
 17. The isolated switching regulator of claim15 further comprising a reset circuit that will cause the isolatedswitching regulator to resoft-start after a fault on the output voltage.18. The isolated switching regulator of claim 16 further comprising areset circuit that will cause the isolated switching regulator toresoft-start after a fault on the output voltage.